This invention relates to a digital control circuit for thyristors which gate an AC waveform to a load.
Control circuits for triggering a thyristor to gate a selected phase angle of an AC waveform to a load are generally analog in nature. The AC waveform is often integrated to produce a ramp-shaped voltage which triggers the thyristor upon reaching a selected level which corresponds to a desired firing angle. The load is often an induction motor which requires AC drive as opposed to DC drive.
Thyristor control circuits for coupling selected phase angles of an AC waveform to an induction motor have been somewhat digital in nature. Typically, however, clock pulses are integrated to generate analog-type control voltages, or clock pulses are directly gated through the thyristor to a load. Precise control of the thyristor firing angle, by use of primarily digital techniques, has not been satisfactorily accomplished. Furthermore, digital control circuits are difficult to adapt to a motor speed control system in which a transducer monitors the shaft speed of a motor.